1. Field of the Invention
The present invention relates to a technique of controlling the density of an image in an electrophotographic image forming apparatus.
2. Description of the Related Art
Conventionally, color image forming apparatuses having individual image carriers generally have a function of automatically controlling image density so as to realize accurate color reproduction and tone stability.
In image density control, a plurality of measurement images (patches) are generally formed on an intermediate transfer member, which is a rotary member, while imaging conditions are being changed. The plurality of patches are then detected using an image density detector provided in an image forming apparatus, the amount of attached toner is calculated based on the detection result, and optimum imaging conditions are determined based on the calculation result.
Also, to obtain optimum values for respective ones of multiple types of imaging conditions, multiple types of image density control are generally performed. Here, examples of imaging conditions include conditions such as charge voltage, exposure intensity, and development voltage, as well as look-up table settings for use in converting an input signal from a host into output image data when a halftone image is to be formed. Tone varies with changes in usage environment, usage histories of various consumables, and so on, and hence, to make tone stable at all times, this image density control needs to be performed on a regular basis.
According to detection principles for optical image density detectors, reflected light from a patch or an intermediate transfer member itself in response to light emitted from a light-emitting device is obtained by a light-receiving device, and based on the result, the amount of toner attached to the patch is calculated. Conversion into the amount of attached toner is actually carried out based on the relationship between an output from the light-receiving device when there is a patch on the intermediate transfer member and an output from the light-receiving device when there is no patch on the intermediate transfer member. This is because reflected light from a patch is affected not only by the amount of toner attached but also by the reflectivity of a surface of the intermediate transfer member.
The reflectivity of a surface of the intermediate transfer member varies according to the position of the surface of the intermediate transfer member. Therefore, in order to accurately calculate the amount of attached toner, an output in a case where there is a patch and an output a case where there is no patch need to be obtained at the same position on the intermediate transfer member. Accordingly, in general, a base output VB from the light-receiving device when there is no patch is obtained at a specific position, and after that, the intermediate transfer member is rotated at least one lap, and a patch is formed at the same position to obtain a patch output VP from the light-receiving device. The base output VB corresponds to reflected light from a base of the intermediate transfer member, and the patch output VP corresponds to reflected light from the patch.
It should be noted that methods to identify the same position on the intermediate transfer member include a method in which a mark provided as a reference is detected, and a method in which the circumferential length of the intermediate transfer member is detected. According to the method in which the circumferential length of the intermediate transfer member is detected, the circumferential length is divided by the circumferential velocity (process speed) of the intermediate transfer member to obtain a time period required for a specific position on the intermediate transfer member to rotate one lap. This time period is counted during rotation, and the timing with which the same position comes again is identified.
When the same position on the intermediate transfer member is to be detected, there is a problem that the circumferential length varies with changes in circumferential parts of the intermediate transfer member, atmospheric environments surrounding the image forming apparatus, and so on. Namely, treating the circumferential length as a fixed value causes an error in position identification. Accordingly, a mark provided as a reference or information on the circumferential length needs to be measured on a regular basis so that positional errors can fall inside an allowable range.
According to Japanese Laid-Open Patent Publication (Kokai) No. 2010-9018, an optical density detector is used to detect reflected light from a surface of an intermediate transfer member to obtain waveform data while the intermediate transfer member is being rotated. Then, matching between waveform data detected in the first lap and waveform data detected in the second lap is performed to identify the same position and calculate information on the circumferential length.
However, according to the prior art, when density adjustment control is performed immediately after the circumferential length is detected, the intermediate transfer member has to be rotated at least one lap so as to obtain waveform data for detection of the circumferential length. Thereafter, the intermediate transfer member is further rotated to make at least one lap so as to obtain an output in a case where there is a patch and an output in a case where there is no patch so as to detect image density. Namely, there is a problem that a time period for rotating the intermediate transfer member at least two laps is required.